Summary: This implements support for explicitly marking the sequence of build steps. Users can now drag and re-order build steps in plans, and artifact dependencies are re-calculated so that if you move "Run Command" before "Lease Host", the "Run Command" step has it's artifact setting cleared and thus the step becomes invalid.
Test Plan: Re-ordered build steps and observed dependencies being correctly recalculated.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T1049
Differential Revision: https://secure.phabricator.com/D7715
